1. Field of the Invention
This invention relates to improvements in or relating to a fabrication method of a field-emission cold cathode for a miniature vacuum field-effect device, that is, a vacuum microelectronic device.
2. Description of the Related Art
Conventionally, various proposals have been made for a structure and a fabrication method of a field-emission cold cathode for a vacuum microelectronic device.
For example, a field-emission cold cathode of the Spindt type disclosed by C. A. Spindt has such a structure as shown in FIG. 1 wherein a plurality of holes are perforated through an insulating layer 32 and a gate electrode 33 deposited on a silicon base 31, and a conical field-emission cathode 34 coated with a film of molybdenum (Mo) formed by vapor deposition is accommodated in each of the holes (IEEE Transaction on Electron Devices, pp. 2,355-2,363, Vol. 38, No. 10, 1991).
The Spindt type cold cathode of FIG. 1 is formed by the following procedure.
First, an oxide film of silicon dioxide (SiO2) is deposited into a thickness of approximately 1 .mu.m on a silicon (Si) substrate 31 of a high specific conductance to form an insulating layer 32, and Mo which is for making gate electrode 33 is deposited into a thickness of 0.25 .mu.m on the insulating layer 32. Then, a pattern of holes of the diameter of approximately 1 .mu.m is formed in an array on the gate electrode 33 by EB exposure (electron-beam exposure). Thereafter, the oxide film of the insulating layer 32 and the Mo film of the gate electrode 33 are etched using this pattern. Further, a sacrifice layer of aluminum (A1) is vapor deposited obliquely on the gate electrode, and Mo is vapor deposited in the holes to form cone-line metal tips of Mo. Finally, the sacrifice layer of Al is removed by etching to form a cold cathode.
Meanwhile, a cold cathode of another structure shown in FIG. 2, which has been published by Urayama et al., may be fabricated by another method wherein the surface of a Si substrate 41 is worked into a trapezoidal shape by isotropic etching and then the Si substrate 41 is thermally oxidized to form a cold cathode cone, whereafter an insulating layer 42 and a gate electrode 43 are successively deposited on the cold cathode cone. In particular, a cap of a circular pattern of SiO.sub.2 of approximately 2 .mu.m is formed as a wet etching mask for Si on a Si substrate 41. Then, the Si substrate 41 is etched into a cone using an alkali etchant of KOH. Thereafter, the surface of the Si substrate 41 is thermally oxidized using an oxidizing furnace while the cap is left on the Si substrate 41 to form an insulating layer 42 of SiO.sub.2 of the thickness of approximately 0.3 .mu.m. Further, a gate electrode 43 of Mo is obliquely vapor deposited into the thickness of approximately 0.3 .mu.m using an EB vapor depositing apparatus. Finally, the SiO.sub.2 film is etched using buffered hydrofluoric acid to produce an electron emitting point of an emitter (M. Urayama, Y. Maruo, Y. Akagi and T. Ise, Fabrication of Cone-like Field Emitters, A Collection of Lecture Drafts for the 53rd Science Lecture Meeting of Applied Physics, 19a-ZM-6, pp.553, The Society of Applied Physics of Japan, Sep. 22, 1992).
The fabrication methods of a Spindt type cold cathode described above are so restricted in conditions for production of a film such as the film formation temperature or the directivity of vapor deposited particles with respect to a substrate that it is difficult to obtain cones of crystal of good quality because an emitter of Mo is formed by vapor deposition by a lift-off method after an insulating layer and a gate electrode are formed. Further, since an Al layer which is a layer to be peeled off is formed by oblique vapor deposition, even if a substrate is rotated and revolved, the vapor deposition condition of Al varies within the substrate, and consequently, a plurality of emitters formed in a plane will exhibit non-uniform shapes. Further, since an end of a cone is formed at the last stage in both of the fabrication methods, they have a problem in that it is difficult to improve the quality of the material of the cone and coat the cone with a different material.